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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::span&lt;T,Extent&gt;::</span>span</h1>
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<tr class="t-nv-h1"><td colspan="5">成员函数</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>span::span</span></span></strong></div></div></td></tr>
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<tr class="t-nv-h2"><td colspan="5">元素访问</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="front.html" title="cpp/container/span/front"><span class="t-lines"><span>span::front</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="back.html" title="cpp/container/span/back"><span class="t-lines"><span>span::back</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="operator_at.html" title="cpp/container/span/operator at"><span class="t-lines"><span>span::operator[]</span></span></a></div></div></td></tr>
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<tr class="t-nv-h2"><td colspan="5">迭代器</td></tr>
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</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> span<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td> (1) </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> It<span class="sy1">&gt;</span><br>
<span class="kw4">constexpr</span> span<span class="br0">(</span>It first, size_type count<span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td> (2) </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> It, <span class="kw1">class</span> End<span class="sy1">&gt;</span><br>
<span class="kw4">constexpr</span> span<span class="br0">(</span>It first, End last<span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td> (3) </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><a href="../../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N<span class="sy1">&gt;</span><br>
<span class="kw4">constexpr</span> span<span class="br0">(</span>element_type <span class="br0">(</span><span class="sy3">&amp;</span>arr<span class="br0">)</span><span class="br0">[</span>N<span class="br0">]</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td> (4) </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> U, <a href="../../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N<span class="sy1">&gt;</span><br>
<span class="kw4">constexpr</span> span<span class="br0">(</span><a href="../array.html"><span class="kw1268">std::<span class="me2">array</span></span></a><span class="sy1">&lt;</span>U, N<span class="sy1">&gt;</span><span class="sy3">&amp;</span> arr<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td> (5) </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> U, <a href="../../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N<span class="sy1">&gt;</span><br>
<span class="kw4">constexpr</span> span<span class="br0">(</span><span class="kw4">const</span> <a href="../array.html"><span class="kw1268">std::<span class="me2">array</span></span></a><span class="sy1">&lt;</span>U, N<span class="sy1">&gt;</span><span class="sy3">&amp;</span> arr<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td> (6) </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> R<span class="sy1">&gt;</span><br>
<span class="kw4">constexpr</span> span<span class="br0">(</span>R<span class="sy3">&amp;&amp;</span> r<span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td> (7) </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> U, <a href="../../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N<span class="sy1">&gt;</span><br>
<span class="kw4">constexpr</span> span<span class="br0">(</span><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">span</span><span class="sy1">&lt;</span>U, N<span class="sy1">&gt;</span><span class="sy3">&amp;</span> s<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td> (8) </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> span<span class="br0">(</span><span class="kw4">const</span> span<span class="sy3">&amp;</span> other<span class="br0">)</span> <span class="kw1">noexcept</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span></span></div></td>
<td> (9) </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>构造 <code>span</code> 。
</p>
<div class="t-li1"><span class="t-li">1)</span> 构造空 span 使其 <span class="t-c"><span class="mw-geshi cpp source-cpp">data<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> nullptr</span></span> 而 <span class="t-c"><span class="mw-geshi cpp source-cpp">size<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span></span></span> 。此重载仅若<span class="t-c"><span class="mw-geshi cpp source-cpp">extent <span class="sy1">==</span> <span class="nu0">0</span> <span class="sy3">||</span> extent <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">dynamic_extent</span></span></span>才参与重载决议。</div>
<div class="t-li1"><span class="t-li">2)</span> 构造作为范围 <code>[first, first + count)</code> 上的视图的 span ；产生的 span 有 <span class="t-c"><span class="mw-geshi cpp source-cpp">data<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">to_address</span><span class="br0">(</span>first<span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">size<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> count</span></span> 。若 <code>[first, first + count)</code> 不是合法范围，若 <code>It</code> 不实际实现 <a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/iterator/contiguous_iterator&amp;action=edit&amp;redlink=1" class="new" title="cpp/iterator/contiguous iterator（页面不存在）"><tt>contiguous_iterator</tt></a> 或若 <span class="t-c"><span class="mw-geshi cpp source-cpp">extent <span class="sy3">!</span><span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">dynamic_extent</span> <span class="sy3">&amp;&amp;</span> count <span class="sy3">!</span><span class="sy1">=</span> extent</span></span> 则行为未定义。这些重载仅若
<dl><dd><ul><li><code>It</code> 满足 <a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/iterator/contiguous_iterator&amp;action=edit&amp;redlink=1" class="new" title="cpp/iterator/contiguous iterator（页面不存在）"><tt>contiguous_iterator</tt></a>
</li><li>从 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1">&lt;</span>It<span class="sy1">&gt;</span></span></span> 到 <span class="t-c"><span class="mw-geshi cpp source-cpp">element_type</span></span> 的转换至多为限定转换才参与重载决议。</li></ul></dd></dl></div>


<div class="t-li1"><span class="t-li">3)</span> 构造作为范围 <code>[first, last)</code> 上的视图的 span ；产生的 span 有 <span class="t-c"><span class="mw-geshi cpp source-cpp">data<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">to_address</span><span class="br0">(</span>first<span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">size<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> last<span class="sy2">-</span>first</span></span> 。若 <code>[first, last)</code> 不是合法范围，若 <code>It</code> 不实际实现 <a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/iterator/contiguous_iterator&amp;action=edit&amp;redlink=1" class="new" title="cpp/iterator/contiguous iterator（页面不存在）"><tt>contiguous_iterator</tt></a> ，若 <code>End</code> 不实际实现 <code>It</code> 的 <a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/iterator/sized_sentinel_for&amp;action=edit&amp;redlink=1" class="new" title="cpp/iterator/sized sentinel for（页面不存在）"><tt>sized_sentinel_for</tt></a> ，或若 <span class="t-c"><span class="mw-geshi cpp source-cpp">extent <span class="sy3">!</span><span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">dynamic_extent</span> <span class="sy3">&amp;&amp;</span> last<span class="sy2">-</span>first <span class="sy3">!</span><span class="sy1">=</span> extent</span></span> 则行为未定义。这些重载仅若
<dl><dd><ul><li> <code>It</code> 满足 <a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/iterator/contiguous_iterator&amp;action=edit&amp;redlink=1" class="new" title="cpp/iterator/contiguous iterator（页面不存在）"><tt>contiguous_iterator</tt></a> ，
</li><li> <code>End</code> 满足 <code>It</code> 的 <a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/iterator/sized_sentinel_for&amp;action=edit&amp;redlink=1" class="new" title="cpp/iterator/sized sentinel for（页面不存在）"><tt>sized_sentinel_for</tt></a> ，
</li><li> 从 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1">&lt;</span>It<span class="sy1">&gt;</span></span></span> 到 <span class="t-c"><span class="mw-geshi cpp source-cpp">element_type</span></span> 的转换至多为限定转换，且
</li><li> <code>End</code> 不可转换成 <span class="t-lc"><a href="../../types/size_t.html" title="cpp/types/size t">std::size_t</a></span>才参与重载决议。</li></ul></dd></dl></div>


<div class="t-li1"><span class="t-li">4-6)</span> 构造作为数组 <code>arr</code> 上的视图的 span ；产生的 span 有 <span class="t-c"><span class="mw-geshi cpp source-cpp">size<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> N</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">data<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <a href="../../iterator/data.html"><span class="kw686">std::<span class="me2">data</span></span></a><span class="br0">(</span>arr<span class="br0">)</span></span></span> 。这些重载仅若<span class="t-c"><span class="mw-geshi cpp source-cpp">extent <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">dynamic_extent</span> <span class="sy3">||</span> N <span class="sy1">==</span> extent</span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 且 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../types/remove_pointer.html"><span class="kw613">std::<span class="me2">remove_pointer_t</span></span></a><span class="sy1">&lt;</span>decltype<span class="br0">(</span><a href="../../iterator/data.html"><span class="kw686">std::<span class="me2">data</span></span></a><span class="br0">(</span>arr<span class="br0">)</span><span class="br0">)</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">[</span><span class="br0">]</span></span></span> 可转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp">element_type <span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">[</span><span class="br0">]</span></span></span> 才参与重载决议。这些构造函数模板决不用于类模板实参推导。</div>
<div class="t-li1"><span class="t-li">7)</span> 构造作为范围 <span class="t-c"><span class="mw-geshi cpp source-cpp">r</span></span> 上的视图的 span ；产生的 span 有 <span class="t-c"><span class="mw-geshi cpp source-cpp">size<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">size</span><span class="br0">(</span>r<span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">data<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">data</span><span class="br0">(</span>r<span class="br0">)</span></span></span> 。
<p>若 <code>R</code> 不实际实现 <a href="../../ranges/contiguous_range.html" title="cpp/ranges/contiguous range"><tt>contiguous_range</tt></a> 与 <a href="../../ranges/sized_range.html" title="cpp/ranges/sized range"><tt>sized_range</tt></a> 或当 <span class="t-c"><span class="mw-geshi cpp source-cpp">element_type</span></span> 为非 const 时 <code>R</code> 不实际实现 <a href="../../ranges/borrowed_range.html" title="cpp/ranges/borrowed range"><tt>borrowed_range</tt></a> 则行为未定义。
这些重载仅若
</p>
<dl><dd><ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">extent <span class="sy1">==</span> dynamic_extent</span></span> ；
</li><li> <code>R</code> 满足 <a href="../../ranges/contiguous_range.html" title="cpp/ranges/contiguous range"><tt>contiguous_range</tt></a> 与 <a href="../../ranges/sized_range.html" title="cpp/ranges/sized range"><tt>sized_range</tt></a> ，
</li><li> <code>R</code> 满足 <a href="../../ranges/borrowed_range.html" title="cpp/ranges/borrowed range"><tt>borrowed_range</tt></a> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../types/is_const.html"><span class="kw495">std::<span class="me2">is_const_v</span></span></a><span class="sy1">&lt;</span>element_type<span class="sy1">&gt;</span></span></span> 为 true
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1">&lt;</span>R<span class="sy1">&gt;</span></span></span> 不是 <code>std::span</code> 的特化，
</li><li> <span class="t-lc">std::remove_cvref_t&lt;R&gt;</span> 不是 <span class="t-lc"><a href="../array.html" title="cpp/container/array">std::array</a></span> 的特化，
</li><li> <span class="t-lc">std::is_array_v&lt;std::remove_cvref_t&lt;R&gt;&gt;</span> 为 false ，且
</li><li> 从 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">range_reference_t</span><span class="sy1">&lt;</span>R<span class="sy1">&gt;</span></span></span> 到 <span class="t-c"><span class="mw-geshi cpp source-cpp">element_type</span></span> 的转换至多为限定转换
</li></ul>
</dd></dl>
才参与重载决议</div>
<div class="t-li1"><span class="t-li">8)</span> 从另一 span <code>s</code> 的转换构造函数；产生的 span 有 <span class="t-c"><span class="mw-geshi cpp source-cpp">size<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> s.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">data<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> s.<span class="me1">data</span><span class="br0">(</span><span class="br0">)</span></span></span> 。此重载仅若<span class="t-c"><span class="mw-geshi cpp source-cpp">extent <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">dynamic_extent</span> <span class="sy3">||</span> N <span class="sy1">==</span> extent</span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 且 <span class="t-c"><span class="mw-geshi cpp source-cpp">U <span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">[</span><span class="br0">]</span></span></span> 可转换为 <span class="t-c"><span class="mw-geshi cpp source-cpp">element_type <span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">[</span><span class="br0">]</span></span></span> 才参与重载决议。</div>
<div class="t-li1"><span class="t-li">9)</span> 默认化的赋值构造函数复制大小与数据指针；产生的 span 有 <span class="t-c"><span class="mw-geshi cpp source-cpp">size<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> other.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">data<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> other.<span class="me1">data</span><span class="br0">(</span><span class="br0">)</span></span></span> 。</div>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0"> 参数 </span></h3>
<table class="t-par-begin">


<tr class="t-par">
<td>  first
</td>
<td> -
</td>
<td>  指向序列首元素的迭代器
</td></tr>
<tr class="t-par">
<td>  count
</td>
<td> -
</td>
<td>  序列的元素数
</td></tr>
<tr class="t-par">
<td>  last
</td>
<td> -
</td>
<td>  指向序列末元素后一位置的迭代器或另一哨位
</td></tr>
<tr class="t-par">
<td>  arr
</td>
<td> -
</td>
<td>  用于构造视图的数组
</td></tr>
<tr class="t-par">
<td>  r
</td>
<td> -
</td>
<td>  用于构造视图的范围
</td></tr>
<tr class="t-par">
<td>  s
</td>
<td> -
</td>
<td>  要转换的另一 span
</td></tr>
<tr class="t-par">
<td>  other
</td>
<td> -
</td>
<td>  要复制的另一 span
</td></tr></table>
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8"> 异常 </span></h3>
<div class="t-li1"><span class="t-li">2)</span> 不抛出。</div>
<div class="t-li1"><span class="t-li">3)</span> 在抛出时 <span class="t-c"><span class="mw-geshi cpp source-cpp">last <span class="sy2">-</span> first</span></span> 抛出其所抛出的异常。</div>
<div class="t-li1"><span class="t-li">7)</span> 在 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">size</span><span class="br0">(</span>r<span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">data</span><span class="br0">(</span>r<span class="br0">)</span></span></span> 抛出时抛出其所抛出的异常。</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85"> 参阅 </span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="data.html" title="cpp/container/span/data"> <span class="t-lines"><span>data</span></span></a></div></div>
</td>
<td>  返回指向元素序列起始的指针 <br> <span class="t-mark">(公开成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="size.html" title="cpp/container/span/size"> <span class="t-lines"><span>size</span></span></a></div></div>
</td>
<td>  返回序列中的元素数 <br> <span class="t-mark">(公开成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="operator%3D.html" title="cpp/container/span/operator="> <span class="t-lines"><span>operator=</span></span></a></div></div>
</td>
<td>  赋值 <code>span</code> <br> <span class="t-mark">(公开成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../../iterator/size.html" title="cpp/iterator/size"> <span class="t-lines"><span>size</span><span>ssize</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div>
</td>
<td>   返回容器或数组的大小 <br> <span class="t-mark">(函数模板)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../../iterator/data.html" title="cpp/iterator/data"> <span class="t-lines"><span>data</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div>
</td>
<td>  获得指向底层数组的指针 <br> <span class="t-mark">(函数模板)</span> </td></tr>
</table>

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